1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method. In particular, the present invention relates to image processing in the case where an image is formed using a plurality of color materials, such as toner or ink, of similar colors.
2. Description of the Related Art
The market demand for high quality printers has been growing year after year, and printers with an increased number of colors in the color materials such as toners as compared to conventional four-color ink or toner printers have been proposed and made into products. For example, there are inkjet printers which form images with six colors of ink, including, in addition to the four general colors, cyan (C), magenta (M), yellow (Y), and black (B), light cyan (c) and light magenta (m) to reduce the grainy effect. In ink of light cyan (c) and light magenta (m), the density of a colorant, such as dye, is low. Such six-color printers have also been proposed in the field of electrophotographic printers as in the inkjet printers.
In six-color printers, data for a color material of low density is generated by decomposing data for a dark color material (ink or toner) such as cyan (C) and magenta (M). To be more precise, in the case of printing an image read by a reader, image processing such as a shading correction, color space conversion, logarithmic conversion, black generation, and color correction is conducted on the read image. Data for each of dark color materials C, M, Y, and K is obtained first. Among these data, data for dark color materials C and M is separated into data for light color materials c and m and data for dark color materials C and M (see Japanese Patent Application Laid-Open No. 2001-290319 and Japanese Patent Application Laid-Open No. 2001-318499, which correspond to U.S. Pat. No. 6,498,910). Furthermore, gamma conversion and MTF (modular transfer function) correction are conducted on the color material data including the dark and light color material data obtained by the color separation. The processed data is used as data for image formation in the printer.
In a color separation for a plurality of color materials of similar colors and with different densities as described above, color data is separated such that the amount of use of each color material is switched. For example, the color separation is conducted using a table in which there is a switching gradation value. In the table, only data for a light color material is output with a value corresponding to an input value when the input value does not reach the switching gradation value, and only data for a dark color material is output with a value corresponding to an input value when the input value exceeds the switching gradation value. In such a case, if, for example, edge enhancement processing is conducted on each of data for dark and light color materials in which a switching point exists, a change in density at the switching point, which is not originally an edge in the image, becomes enhanced. As a result, a density gap can appear on the image to be printed. In addition, in the case where an under color removal process is conducted on each of data for dark and light color materials, data for densities lower than a certain level is eliminated. As a result, a section in which there is a drastic change in density can be generated in each data. Such a section in which there is a density change can appear as a density gap in the resulting printed image. These density gaps can degrade the quality of a printed image.
Furthermore, even if there is no apparent switching point or section in which there is a change in density as mentioned above, a density gap or a density nonuniformity can be generated in the case where a gradation changing process other than an edge enhancement or a background color removal mentioned above is conducted on each of dark and light color material data.